The present invention relates to a method of preparing a synthetic latex suited for a method of preparing an industrially useful diene rubber polymer latex etc., which has improved productivity.
Diene rubber polymers are widely known as elastic materials used in ABS and MBS resins and are usually prepared by emulsion polymerization via an aqueous dispersion of a polymer, i.e. synthetic latex. Regarding these diene rubber polymers, a further improvement in productivity is required because of their high utility.
The reaction pressure during the polymerization often reaches high pressures above 0.098 MpaG in the preparation of synthetic latexes represented by diene rubber polymer latex, regardless of the preparation method. Therefore, there arises the problem of controlling the temperature and pressure in the reactor when runaway reactions generate.
Japanese Examined Patent Application, Second Publication No. Sho 54-3511 discloses an improved batchwise preparation method for the purpose of improving control of the polymerization temperature of a batchwise preparation method.
All of the monomer as a raw material is charged in advance in the reactor in case of the batchwise preparation method, whereas the improved batchwise preparation method is a method of allowing the polymerization to proceed while adding dropwise a portion or all of the monomer raw material during the polymerization. In the improved batchwise preparation method, a state in which there is no monomer in the reactor can be attained by stopping the dropwise addition of the monomer, even if it becomes impossible to control the polymerization temperature during the dropwise addition of the monomer. As a result, the reaction rate is reduced to control the polymerization temperature, thereby making it possible to prevent runaway reactions.
According to the method described in Japanese Examined Patent Application, Second Publication No. Sho 54-3511, the polymerization is performed while adding dropwise the total amount of a liquid containing water as a principal component and a monomer as a raw material in a reactor fixed to a high activity under high temperature conditions, continuously or intermittently. In this case, since the total amount of the monomer is added dropwise, excess monomer is not retained in the polymerization system at any time. Therefore, it is easy to control the temperature and pressure in the reactor from the initial stage to the completion of the polymerization.
The present inventors have studied ways to improve the productivity of a diene rubber polymer and suggested a method of preparing a synthetic latex utilizing an improved batchwise preparation method in Japanese Patent Application No. Hei 10-053672.
The technique described in Japanese Patent Application No. Hei 10-053672 is directed to a method of charging a portion of a raw material, polymerizing the charged monomer, thereby to consume the monomer, and allowing the polymerization to proceed while adding dropwise the remainder of the monomer. More specifically, the technique utilizes the sensible heat of water by adding dropwise the monomer and water by half or more of the total amount during the emulsion polymerization, thereby remarkably improving the productivity.
However, according to the technique described in Japanese Unexamined Patent Application, Second Publication No. Sho 54-3511, when a condenser is provided for the purpose of removing heat generated during the polymerization, the heat removal efficiency of the condenser at the initial stage of the polymerization is lowered if some monomer is not present in the reactor before the polymerization.
If the particle diameter of the latex is controlled by using an emulsifier which has already been charged before the polymerization, the particle diameter varies sometimes when the total amount of the monomer is added dropwise. Therefore, an improvement is required.
To reduce scatter in the particle diameter of the latex in the improved batchwise preparation method, it is necessary to charge an emulsifier in the amount according to the desired particle diameter and a monomer in the amount required for the formation of particles. Therefore, the concentration of the emulsifier to be charged generally becomes larger than that in case of the batchwise preparation method by the volume of water to be added dropwise.
With the increase of the concentration of the emulsifier, the polymerization rate becomes higher and the polymerization heating value per unit volume becomes drastically larger. Therefore, the temperature also increases drastically during the runaway reaction, which sometimes makes it difficult to control the temperature.
In the method described in Japanese Patent Application No. Hei 10-053672, since a large amount of the monomer is present in the polymerization system until the charged monomer is consumed, there is a possibility of the occurrence of a runaway reaction. When a large amount of the monomer is present in the reactor, the pressure in the reactor increases with the increase of the polymerization temperature. According to the present inventors"" study, for a diene monomer, the pressure in the reactor increases with the increase of the inner temperature until the conversion ratio based on the charged monomer becomes about 60%. Therefore, a method of controlling runaway reactions during while the pressure increases is separately required.
However, when the amount of water to be charged before the polymerization is reduced to the limit in expectation of an improvement in productivity due to the sensible heat of water, the temperature at the initial stage of the polymerization drastically increases as compared with a conventional batchwise preparation method, which sometimes makes it difficult to control the temperature.
To control the temperature and pressure in the reactor until the charged monomer is consumed, there can be suggested a method of pressurizing a large amount of a polymerization inhibitor using an inert gas such as nitrogen etc. under a pressure, which is sufficiently below the practical proof pressure of the reactor, and injecting it into the reactor.
The practical proof pressure refers to a value which is set so that a pressure change during ordinary operation can be absorbed, in contrast to the proof pressure of the reactor set by the manufacturer, and it varies depending on the kind of the reactor. Usually, there is set an upper limit of the pressure in the reactor, at which a safety valve or a bursting plate provided on the reactor does not operate during ordinary operation.
However, when the above method does not work effectively for some reason and the pressure exceeds the practical proof pressure, the safety valve or bursting plate operates as a final safety device. However, since a large amount of the monomer in the rector is discharged out of the system when the safety valve or bursting plate operates, such a situation should be avoided as much as possible.
As described above, it is generally difficult for the improved batchwise preparation method to control the temperature and pressure at the initial stage of the polymerization reaction, so that control of a runaway reaction is indispensable not only in laboratory-scale preparation but also in the factory scale preparation, and the conventional method has been insufficient.
This problem is not limited to the case of the diene rubber polymer and is common to the method of preparing a synthetic latex due to the emulsion polymerization wherein the reaction pressure during the polymerization reaches a high pressure, higher than 0.098 MpaG.
The present invention has been made in light of the circumstances described above and an object thereof is to improve the productivity by controlling an increase in temperature and pressure in a reactor due to the runaway reaction in all processes in the emulsion polymerization of a synthetic latex. An object of the present invention is to improve the productivity by controlling an increase in temperature and pressure in a reactor due to a runaway reaction until the charged monomer is polymerized, which has to a large extent not been attained, heretofore, by an improved batchwise preparation method.
Another object of the present invention is to improve the productivity in the preparation of an industrially useful diene rubber polymer latex.
The present inventors found that the pressure in the reactor can be controlled by controlling the ratio of monomer/water to be charged in an improved batchwise method of preparing a synthetic latex. At the initial stage of the polymerization where an increase in temperature and pressure in the reactor due to the runaway reaction cannot be controlled sufficiently by a conventional improved batchwise preparation method, the pressure in the reactor was controlled within the practical proof pressure, thus completing a method of preparing a synthetic latex of the present invention, which can stably perform the polymerization reaction.
To solve the problems described above, the present invention suggests a method of preparing a synthetic latex, which comprises charging a portion of water and a monomer as raw materials in a reactor, initiating the polymerization, adding dropwise the remainder of said water and said monomer during the polymerization, continuously or intermittently, and performing the emulsion polymerization, wherein: said monomer and said water are charged in said reactor in a ratio of monomer/water, at which a maximum ultimate pressure in said reactor that is attained if an initial composition in said reactor before the initiation of the polymerization is polymerized from a polymerization initiation temperature in an insulated state, is within a practical proof pressure of said reactor.
This method exerts a large effect on the high pressure emulsion polymerization wherein the reaction pressure during the polymerization is 0.098 MpaG or higher.
If the monomer contains a diene monomer, the method can be applied to preparation of an industrially useful diene rubber polymer latex.
The maximum ultimate pressure can be determined by simulated calculation using the relationship between the conversion ratio and the pressure in the reactor and the relationship between the conversion ratio and the polymerization rate, which are obtained from the test results of a batchwise preparation method in which the polymerization temperature after heating is constant.
As used herein, the term xe2x80x9cinitial compositionxe2x80x9d refers to a composition which is present in a reactor before the charged monomer initiates the polymerization, and is not limited to those to be charged before heating and also includes those to be added during the heating.